Calibrated internal rc oscillator, Atmega169v/l – Rainbow Electronics Atmega169L User Manual
Page 27
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ATmega169V/L
2514A–AVR–08/02
Note:
1. These options should only be used if frequency stability at start-up is not important for
the application
Calibrated Internal RC
Oscillator
The calibrated internal RC Oscillator provides a fixed 8.0 MHz clock. The frequency is
nominal value at 3V and 25
°C. If 8 MHz frequency exceeds the specification of the
device (depends on V
CC
), the CKDIV8 Fuse must be programmed in order to divide the
internal frequency by 8 during start-up. The device is shipped with the CKDIV8 Fuse
programmed. See “System Clock Prescaler” on page 30. for more details. This clock
may be selected as the system clock by programming the CKSEL Fuses as shown in
Table 8. If selected, it will operate with no external components. During reset, hardware
loads the calibration byte into the OSCCAL Register and thereby automatically cali-
brates the RC Oscillator. At 3V and 25
°C, this calibration gives a frequency within ± 1%
of the nominal frequency. When this Oscillator is used as the chip clock, the Watchdog
Oscillator will still be used for the Watchdog Timer and for the Reset Time-out. For more
information on the pre-programmed calibration value, see the section “Calibration Byte”
on page 268.
Note:
1. The device is shipped with this option selected.
When this Oscillator is selected, start-up times are determined by the SUT Fuses as
shown in Table 9.
Note:
1. The device is shipped with this option selected.
Table 7. Start-up Times for the Low-frequency Crystal Oscillator Clock Selection
CKSEL3..0
Start-up Time from
Power-down and Power-save
Recommended Usage
0100
1K CK
0101
32K CK
Stable frequency at start-up
0110
1K CK
0111
32K CK
Stable frequency at start-up
Table 8. Internal Calibrated RC Oscillator Operating Modes
(1)
CKSEL3..0
Nominal Frequency
0010
8.0 MHz
Table 9. Start-up times for the internal calibrated RC Oscillator clock selection
SUT1..0
Start-up Time from Power-
down and Power-save
Additional Delay from
Reset (V
CC
= 5.0V)
Recommended Usage
00
6 CK
14CK
BOD enabled
01
6 CK
14CK + 4.1 ms
Fast rising power
10
(1)
6 CK
14CK + 65 ms
Slowly rising power
11
Reserved